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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

386

FATTY SUBSTANCES  (GENERAL METHODS)

the fact that the ammonium salts of the solid acids are insoluble, and those
of the liquid acids soluble, in alcohol, and (2) that of Fachini and Dorta,1
based on the insolubility of the solid fatty acids and their alkali salts in
cold acetone ; this solvent also serves to separate stearic and palmitic
acids from myristic and lauric acids.

The content of liquid and solid acids in a fatty substance may also be
deduced from the absolute and relative iodine numbers. If /,. is the relative
iodine number or that of the fat as such, and Ia the absolute iodine number
or that of the liquid fatty acids extracted from the fat, I part by weight of

iodine corresponds with  parts by weight of liquid acids (since Ia corre-

*a

sponds with 100 parts by weight of liquid acids). With the quantity of
iodine Ir absorbed by 100 parts of the fat there correspond ------? parts of

* a

liquid fatty acids, this last fraction giving the percentage of liquid fatty
acids in the fatty substance.

When a mixture of palmitic, stearic and oleic acids along is deal t with,
the content of oleic acid may be calculated from the relative iodine number
of the mixture, since it is known that the theoretical iodine number of oleic
acid is 90-07. Thus, if J is the relative iodine number of the mixture and
0 the percentage of oleic acid sought,

0 = -

100 X /

90-07

or 0  I-H02/.

Lastly, it must be pointed out that, with mixtures of the three acids
named above, the content of liquid and solid acids may be determined by
means of the solidification point of the mixture, use being, made of Dalican's
table (see Tallow and Stearine, i).

2.  Determination of the Stearic Acid.The content of stearic acid
in a mixture of fatty acids obtained by saponification of a fat may be deter-
mined by Hehner and Mitchell's method, which is based on the fact that
stearic acid is very slightly soluble in alcohol at o, whilst palmitic acid is
much more soluble and the liquid acids readily soluble.

From 0-5 to i gram of the fatty acids, if solid, or 5 grams if liquid, are
dissolved in 100 c.c. of alcohol of D = 0-8183 (94'4% alcohol) already
saturated at o with pure stearic acid; the solution is then cooled to o
and filtered and the residue washed with alcohol saturated with stearic
acid, working always at o; finally the stearic acid remaining insoluble is
weighed.

This method is applicable especially to mixtures of stearic acid with palmitic
and oleic acids, which are the most common ; with more complex mixtures
it does not give satisfactory results.2

3.  Determination of the Stearic and  Palmitic Acids.With a
mixture of solid fatty acids free from liquid acids and composed, as is the

1  Rend. Soc. chim. ital., 1912, p. 51.

2  On the solubility of the various acids in alcohol, see also a paper by Kreis and
Hafner in Zeii. Unt. Nahr. und Genussmittel, 1903, p. 22, and also papers by Heiduschka
and Burger and by Serger in Zeits. fur offent. Chem., 1913, pp. 87 and 131.hus separated may then be utilised for other investigations,